Photosensitive integrated circuit equipped with a reflective layer and corresponding method of production

ABSTRACT

A method for producing a photosensitive integrated circuit including producing circuit control transistors, producing, above the control transistors, and between at least one upper electrode and at least one lower electrode, at least one photodiode, by amorphous silicon layers into which photons from incident electromagnetic radiation are absorbed, producing at least one passivation layer, between the lower electrode and the control transistors, and producing, between the control transistors and the external surface of the integrated circuit, a reflective layer capable of reflecting photons not absorbed by the amorphous silicon layers.

This application claims priority from French patent application No. 0511755, filed Nov. 21, 2005, which is incorporated herein by reference.

TECHNICAL FIELD

An embodiment relates to the field of the production of microchips. More specifically, an embodiment relates to the first phases in the production of a chip, in particular phases in the production of integrated circuits.

BACKGROUND

Chips are used in an increasing number of products for mainstream and professional use. Also, the development of electronic devices integrating optical products such as “webcam” cameras, photo or 3G mobile telephones, optical mice, and so on, requires the development of photosensitive chips.

SUMMARY

An embodiment relates specifically to these photosensitive chips, the photosensitivity being provided by photodiodes.

A photodiode is a junction diode designed to be used as a photodetector by exposing one side of the junction to optical radiation. This junction is intended to be illuminated by a window formed in the protective oxide layer of the chip and possibly placed under a microlens. The light that reaches the junction causes the current to increase proportionally to the illumination thereof, as described below.

Accordingly, an embodiment relates to a method for producing a photosensitive integrated circuit including the steps of:

-   -   producing circuit control transistors,     -   producing, above the control transistors, and between at least         one upper electrode and at least one lower electrode, at least         one photodiode, by amorphous silicon layers into which photons         from incident electromagnetic radiation are absorbed, and     -   producing at least one passivation layer, between the lower         electrode and the control transistors.

The photodiode is configured so as to convert incident electromagnetic radiation into a current by the absorption of photons.

If the energy of the incident electromagnetic radiation is adequate, the photons are absorbed in the amorphous silicon layer. An electron of the silicon then goes from the valence band to the conduction band, creating an electron-hole pair.

Typically, a photodiode is constituted by a deposit of both n- and p-doped amorphous silicon (one n-doped layer and one p-doped layer). The n and p portions are respectively connected to a circuit by means of an electrode array and a transparent electrode. When the junction is illuminated, the light absorbed there creates electron-hole pairs. The free electrons are drawn to the n portion and the holes are drawn to the p portion. The current (or the difference in potential) measured at the terminals of the junction is proportional, within a certain limit, to the illumination of the photodiode.

However, the photons of the incident radiation may not all be absorbed. And, depending on the illumination conditions, some light rays may not be absorbed by the amorphous silicon of the junction.

If the incidence of the rays occurs on a lower electrode of the electrode array, the rays are reflected and may be sent to the amorphous silicon.

However, if the incidence of the rays occurs between two lower electrodes, the rays are refracted, then absorbed by the intermetallic levels of the integrated circuit.

Also, by being absorbed under the photodiodes, these rays are no longer involved in the creation of electron-hole pairs, and are, therefore, lost to the device. This undesired absorption process reduces the quantum efficacy of the system.

The quantum efficacy is defined by the fraction of incident photons involved in the photocurrent and is dependent on the wavelength of the incident radiation.

An embodiment is intended to overcome the aforementioned disadvantages by proposing a solution designed to improve the quantum efficacy.

Therefore, a method according to an embodiment, which is otherwise consistent with the preamble above, also includes producing, between the control transistors and the external surface of the integrated circuit, a reflective layer capable of reflecting photons not absorbed by the amorphous silicon layers.

Owing to this arrangement, a ray that has not been absorbed by the amorphous silicon is reflected onto the reflective layer. It is thus returned to the amorphous silicon layer and can again generate an electron-hole pair.

In an embodiment of the method for producing an integrated circuit according to the invention, the reflective layer is inserted between a passivation layer and the amorphous silicon layers.

The reflective layer, like each of the layers of the integrated circuit, may be produced by chemical vapor deposition (CVD). However, other techniques can be envisaged, such as SOL-GEL, SPIN-ON, ALD or PVD techniques. An oxidizing or non-oxidizing post-deposition annealing step can also be performed.

The reflective layer is advantageously produced from a white material such as a metal oxide, and preferably titanium oxide, aluminum oxide, zirconium oxide or magnesium oxide.

An embodiment also relates to a photosensitive integrated circuit including at least one photodiode configured to convert incident electromagnetic radiation into a current by the absorption of photons into at least one amorphous silicon layer by the creation of electron-hole pairs.

The integrated circuit according to an embodiment also includes a reflective layer capable of reflecting, inside the integrated circuit, the unabsorbed photons.

In an embodiment, the reflective layer is located just below the photodiode(s), inserted between a passivation layer and the amorphous silicon layers.

Also, the reflective layer is a white material produced from a metal oxide, the metal being selected from the group including at least titanium, aluminum, zirconium and magnesium.

In addition, the reflective layer has a reflection surface of which the roughness is chosen so as to produce a specular or diffuse reflection.

Finally, an embodiment relates to an electronic device including the integrated circuit as described.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become clearer from the following description, given by way of a non-limiting example, in reference to the single appended figure.

FIG. 1 shows a transversal cross-section of an integrated circuit according to an embodiment.

DETAILED DESCRIPTION

This embodiment is advantageously implemented in microchips satisfying standards CMOS180, CMOS120, CMOS090 or CMOS065. It can also be implemented in any other embodiments such as the following embodiments: a first embodiment in which the photodiodes (the photosensitive device) are in “back-end”, i.e., in the upper levels of the integrated circuit; or a second embodiment in which the photodiodes (the photosensitive device) are in “front-end”, i.e., in the lower levels of the integrated circuit.

These standards correspond to the gate widths of the transistors of the circuit, with standard CMOS065 corresponding to the smallest current technology, that is 65 nm.

For the sake of clarity, only the embodiment in which the photodiodes are in “back-end” will be described below. This embodiment is sometimes preferred because the presence of photodiodes in the upper levels of the integrated circuit often enables a maximum amount of light to be collected and transformed.

As shown in FIG. 1, a photosensitive integrated circuit includes at least one photodiode. The photodiode includes, for example, two amorphous silicon layers of which one 5 is p-doped and the other 6 is n-doped.

The p-doped layer is connected to a circuit by means of an upper electrode 4 connected to an electrode E1. The n-doped layer is connected to a circuit by means of a lower electrode E2.

The upper electrode 4 is continuous, i.e., it has a continuous contact surface with the p-doped amorphous silicon layer of the photodiode.

The upper transparent electrode 4 may be constituted by an indium-doped tin oxide.

However, the lower electrode is discontinuous and is constituted, as shown in the single figure, by a number of electrodes E2, separated from one another.

Each lower electrode E2 may be constituted by chromium or a chromium-based material.

In an embodiment, the circuit includes, above the upper electrode 4, one or more filters 3. These filters, for example colored, allow in this case for the separation of the incident light (hv) into basic components (e.g., red, green, blue).

The circuit may also include, on the side of its external surface, a planarizing layer 2 produced, for example, by deposition of a resin, and intended to level out the topography of the circuit associated with the production of colored filters with resins of various thicknesses.

In an embodiment, the circuit includes microlenses 1 positioned on the external surface of the circuit.

These microlenses make it possible to focus the incident radiation on the lower electrodes E2, each microlens being normally or otherwise optically aligned with a lower electrode, so as to optimize the quantum efficacy (the microlens 1 may be shifted relative to the respective electrodes E2 in outer regions of the array to account for the angle of the incident light).

Under the photodiodes, the circuit includes a passivation layer. This passivation layer can be separated into two layers, typically one 8 being made of a material such as silicon nitride and the other 9 being made of a material such as a phosphorus-doped oxide (PSG).

Under these passivation layers is an intermetallic insulation layer 10, typically an oxide.

Finally, under this intermetallic insulation layer are the circuit control transistors (not shown).

At the electronic level, each electrode E2 is connected to a conductive element 14, such as metal, for example tungsten or copper, said element 14 itself being connected to another conductor 12, such as metal, for example, aluminum or copper.

These elements are connected two-by-two by a diffusion barrier (11, 13, 15), for example a titanium/titanium nitride alloy, of which at least one 11 has anti-reflective properties.

The circuit also has a reflective layer 7. In an embodiment, this layer is a white material made from a metal oxide in which the metal of the metal oxide is chosen from the group including at least titanium, aluminum, zirconium and magnesium.

Owing to this reflective layer, if a photon hv, of which the trajectory is shown by a wavy arrow in the single figure, is not absorbed by the amorphous silicon layers of the photodiode during its first incidence, it is reflected and can therefore again generate an electron-hole pair, which increases the quantum efficacy of the device.

Similarly, in the case in which the circuit comprises microlenses, it is possible that the lenses will not be suitably aligned with respect to the lower electrodes. In this case, the circuit makes it possible to compensate for this defect by sending the unfocused photons to the amorphous silicon, thus again improving the quantum efficacy of the device.

In addition, to limit the negative impact of the external medium (heat, radiation, etc.) on the integrated circuits, the latter are generally protected in opaque housings. This mode of protection typically cannot be used as such for photosensitive integrated circuits.

Moreover, under strong illumination, the performance of electronic devices including photosensitive integrated circuits can be significantly hindered due to the generation of charges in the doped areas of the transistors.

These stored charges may cause parasitic phenomena such as:

-   -   the increase in background noise of the detector (increase in         the dark current), and     -   the variation in control voltages of the transistors (unique         behavior of one transistor with respect to another).

But the reflective layer 7 may limit these negative effects by protecting the control transistors from these so-called “stacking” effects.

Finally, the integration of a reflective and protective layer may make it possible to obtain more dynamic sensors (better response between weak and strong illumination).

Furthermore, the metal of the metal oxide used in the production of the reflective layer may be chosen on the basis of the anticipated wavelength of the incident radiation, as the reflective properties of a material may vary with wavelength.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. 

1. Method for producing a photosensitive integrated circuit comprising: producing circuit control transistors, producing, above the control transistors, and between at least one upper electrode and at least one lower electrode, at least one photodiode, by amorphous silicon layers into which photons from incident electromagnetic radiation are absorbed, producing at least one passivation layer, between the lower electrode and the control transistors, and producing a reflective layer capable of reflecting photons not absorbed by the amorphous silicon layers just below the photodiode(s), inserted between a passivation layer and the amorphous silicon layers.
 2. Method for producing an integrated circuit according to claim 1, in which the reflective layer comprises a white material made from a metal oxide, the metal of the metal oxide being selected from the group including at least titanium, aluminum, zirconium and magnesium.
 3. Method for producing an integrated circuit according to claim 1, also including producing at least one microlens on the external surface of the integrated circuit, configured so as to focus the incident electromagnetic radiation toward at least one lower electrode.
 4. Photosensitive integrated circuit including at least one photodiode configured so as to convert incident electromagnetic radiation into a current by the absorption of photons into at least one amorphous silicon layer by the creation of electron-hole pairs, further including a reflective layer capable of reflecting, inside the integrated circuit, the unabsorbed photons, said reflective layer being located just below the photodiode(s), inserted between a passivation layer and the amorphous silicon layers.
 5. Photosensitive integrated circuit according to claim 4, in which the reflective layer comprises a white material made from a metal oxide.
 6. Photosensitive integrated circuit according to claim 5, in which the metal of the metal oxide is chosen from the group including at least titanium, aluminum, zirconium and magnesium.
 7. Photosensitive integrated circuit according to claim 4, in which the reflective layer has a reflection surface of which the roughness is chosen so as to produce a specular or diffuse reflection.
 8. A semiconductor structure, comprising: a photodiode having an electromagnetic-energy-receiving first surface and a second surface that is opposite to the first surface; and a reflector disposed adjacent to the second surface of the photodiode.
 9. The semiconductor structure of claim 8 wherein the photodiode comprises: a first semiconductor layer having a first conductivity type, a first surface that forms the first surface of the photodiode, and a second surface opposite to the first surface; and a second semiconductor layer having a second conductivity type, a first surface contiguous with the second surface of the first semiconductor layer, and a second surface opposite to the first surface and that forms the second surface of the photo diode.
 10. The semiconductor structure of claim 8 wherein the reflector comprises a white material that comprises a metal oxide.
 11. The semiconductor structure of claim 8 wherein the reflector is contiguous with the second surface of the photodiode.
 12. The semiconductor structure of claim 8, further comprising: a transparent first electrode contiguous with a portion of the first surface of the photodiode; and a second electrode contiguous with a portion of the second surface of the photodiode.
 13. The semiconductor structure of claim 8 wherein the reflector comprises a rough surface that is adjacent to the second surface of the photodiode.
 14. The semiconductor structure of claim 8 wherein the photodiode comprises: a first amorphous silicon layer having a first conductivity type, a first surface that forms the first surface of the photodiode, and a second surface opposite to the first surface; and a second amorphous silicon layer having a second conductivity type, a first surface contiguous with the second surface of the first amorphous silicon layer, and a second surface opposite to the first surface and that forms the second surface of the photo diode.
 15. An integrated circuit, comprising: an array of photodiodes each having a respective electromagnetic-energy-receiving first surface and a respective second surface that is opposite to the first surface; and a respective reflector disposed adjacent to the second surface of each photodiode.
 16. The integrated circuit of claim 15 wherein: the respective first surfaces of the photodiodes compose a single surface; and the respective reflectors compose a single reflector.
 17. The integrated circuit of claim 15, further comprising: a transparent first electrode contiguous with a respective portion of each of the first surfaces of the photodiodes; and respective second electrodes each contiguous with a portion of the second surface of a respective photodiode.
 18. The integrated circuit of claim 15, further comprising an array of color filters disposed over the array of photodiodes, each filter optically aligned with a respective one of the photodiodes.
 19. The integrated circuit of claim 18 wherein: a first number of the filters attenuate blue and green light relative to red light; a second number of the filters attenuate red and blue light relative to green light; and a third number of the filters attenuate red and green light relative to blue light.
 20. The integrated circuit of claim 15, further comprising an array of microlenses disposed over the array of photodiodes, each microlens optically aligned with a respective one of the photodiodes.
 21. The integrated circuit of claim 15, further comprising: wherein the respective first surfaces of the photodiodes compose a single first surface; wherein the respective reflectors compose a single reflective layer; a transparent first electrode contiguous with the single first surface of the photodiodes; an array of color filters disposed over the transparent first electrode, each filter optically aligned with a respective one of the photodiodes, some of the filters having different heights relative to others of the filters; a transparent layer disposed over the filters and having a planar surface opposite from the filters; and an array of microlenses disposed over the transparent layer, each microlens optically aligned with a respective one of the color filters and a respective one of the photodiodes.
 22. An electronic system, comprising: an integrated circuit, comprising, an array of photodiodes each having a respective electromagnetic-energy-receiving first surface and a respective second surface that is opposite to the first surface, and a respective reflector disposed adjacent to the second surface of each photodiode; and a controller coupled to the integrated circuit.
 23. A method, comprising: receiving with a photodiode electromagnetic energy incident on a first surface of the photodiode and propagating toward a second surface of the photodiode that is opposite to the first surface; and redirecting the electromagnetic energy back toward the first surface.
 24. The method of claim 23 wherein the electromagnetic energy comprises energy within a portion of the visible spectrum.
 25. The method of claim 23 wherein the electromagnetic energy comprises light.
 26. The method of claim 23 wherein redirecting comprises redirecting the electromagnetic energy substantially at the second surface of the photodiode.
 27. The method of claim 23 wherein redirecting comprises redirecting the electromagnetic energy at a location that is beyond the second surface of the photodiode such that the second surface is between the location and the first surface of the photodiode.
 28. The method of claim 23 wherein redirecting comprises redirecting the electromagnetic energy at a location that is between the first and second surfaces of the photodiode. 